I always find slow cooker gravy watery. Just before you are ready to serve put the gravy into a pan and thicken in whatever way you usually would. If the meat is OK but the veg are still crisp, just serve the meat and cook some other veg perhaps?

maybe should have been on high? if watery mix some cornflour with little bit of cold water and add; or add gravy granules. carrots can be a bit pesty - need to be fairly small. add mustard if gravy bit bland.

make a [[ http://en.wikipedia.org/wiki/Beurre_mani%C3%A9 buerre manie,]] Mash some flour into butter then let it melt into the gravy. Also you can get some thickening granules which are good to add at the end of the cooking

Slow cooking always ends up more runny than if you do stuff in the oven/on hob. I tend to use cornflour to thicken it. I'm usually in so add it about 3pm to eat at 6pm but a couple of hours should be enough. I also find onions never really cook well enough in mine so I either fry them before sticking them in or use something else instead (usually leek cos we usually have some in the fridge) and I cut my carrots smaller than usual too.